1. Field of the Invention
This invention relates generally to methods for the site-directed activation of prodrugs. More particularly, the invention relates to the administration of a prodrug in conjunction with a site-directed catalytic antibody which converts the prodrug to an active drug.
2. Description of the Background Art
Many efforts have been made to increase the selectivity of drugs by creating prodrug versions whereby the activity of the drug is attenuated by some pendent group. For a review of these efforts, see Bundgaard, H., ed., (1985) Design of Prodrugs, Elsevier Science Publishers, Amsterdam. The prodrug is envisioned to undergo either spontaneous degradation or, preferably, enzymatic conversion to the active form. Theoretically, selectivity can be increased by relying on an enzyme that is over-produced by, or is only present at, the target cells. However, despite the appeal of this approach, this strategy has not proven widely successful, primarily due to the difficulty of identifying enzymes specific to the target cells.
Attempts have also been directed to site specific drug delivery utilizing monoclonal antibodies. These drug delivery systems employ a cytotoxic agent which has been conjugated directly to an antibody recognizing the desired antigen. Utilizing this method, radioisotopes, drugs, and biotoxins have been conjugated to monoclonal antibodies, and selective cytotoxicity has been observed. This method is described generally by Borrebaeck et al. (Eds.) (1990) Therapeutic Monoclonal Antibodies, Stockton Press: New York.
The use of antibody-targeted site specific drug delivery suffers from many drawbacks. The most obvious PG,3 shortcoming results from the heterogeneity in distribution of the antigen amongst targeted cells. Not all target cells may produce the antigen, thus, allowing some target cells to escape. Moreover, cells in poorly vascularized sites, such as in a tumor mass, are virtually inaccessible to the antibody. Thus, these cells will also escape treatment. Further, the slow clearance of antibodies from the body, with half-lives typically on the order of a few days, contributes to cytotoxic effects in normal cells.
In addition, the use of antibody-targeted drugs relies on endocytosis of the conjugate, followed by degradation in the lysosomes to release the drug inside the targeted cells. The rates of internalization of the conjugates by various cancer cells have been studied (Killion, et al. (1989) Seminars in Oncology, 16:106-115) and shown to be highly variable, with some cells exhibiting minimal uptake of the conjugates. Finally, the acidic pH of the lysosomes limits this technique to acid stable drugs.
A further approach to site specific drug delivery utilizes antibody directed enzymes. Antibodies which are capable of recognizing the target cells are coupled to enzymes which are capable of unmasking a prodrug. After localization of the enzyme/antibody conjugate at the target cells, the prodrug is administered and rendered cytotoxic at the target site. This approach has been described by Bagshawe et al. (1988) Br. J. Cancer, 58:100; and Bagshawe (1987) Br. J. Cancer, 56:531.
The antibody directed enzyme approach has been expanded upon by Haber et al. European Patent Pub. No. 0,187,658. Haber describes the site-specific activation of an inactive toxic agent by an antibody bound to an activator. The activator activates the inactive toxic agent by reacting chemically, allosterically or enzymatically with the inactive substance.
Powell et al. PCT patent publication No. WO89/10754 has suggested that catalytic antibodies may be "site specific" in that they are deliberately designed only to catalyze cleavage of bonds having certain structural conformations at specific sites in a biomolecule. Powell does not describe a protocol for targeted drug release at specific target sites
The preparation of catalytic antibodies against haptens that are transition state analogs is described in the following references: Pollack et al. (1986) Science, 234:1570-1573; Pollack and Schultz (1987) Cold Spring Harbor Symp. Quant. Biol., 52:97-104; Pollack and Schultz (1989) J. Am. Chem. Soc., 111:1929-1931; Jacobs et al. (1987) J. Am. Chem. Soc., 109:2174-2176; Tramontano et al. (1986) Science, 234:1566-1570; Tramontano et al. (1988) J. Am. Chem. Soc., 110:2282-2286; Janda et al. (1988) Science, 241:1188-1191; Schochetman et al. (1989) U.S. Pat. No. 4,888,281; Benkovic et al. (1990) U.S. Pat. No. 4,900,674; Napper et al. (1987) Science 237:1041-43; European Patent Pub. No. 0,251,093; and Paul et al. PCT patent publication No. 91/14769.
The use of catalytic antibodies to overcome entropic barriers involved in orienting reaction partners is described in the following references: Jackson et al. (1988) J. Am. Chem. Soc., 110:4841-4842; Janda et al. (1988) J. Am. Chem. Soc., 110:4835-4837; Hilvert et al. (1988) Proc. Natl. Acad. Sci. USA, 85:4953-4955; and Benkovic et al. (1988) Proc. Natl. Acad. Sci. USA, 85:5355-5358.
The disclosures of the above cited references and other references referred to throughout this application are incorporated herein by reference.